Narcissists' relationship with sex is as empty and meaningless as many of their other pursuits in life. In general, being a narcissist is an endless loop of finding sources of "narcissistic supply": acting effectively as a psychological vampire on others in order to seek emotional nourishment. Except that the type of "nourishment" that a narcissist seeks is entirely in one direction - their own.
In this way, narcissists and (more socially dangerous) psychopaths share some common characteristics. Both these types of individuals lack empathy, and seek to use others in society solely for what they can get out of them. Psychopaths are also narcissists - some may well be called "psychopathic narcissists". A psychopath and a narcissist therefore share the same root perception of what "sex" means - more on the dangerous relationship that psychopaths have with sex can be found here. A psychopath, however, is much more dangerous to be around.
To be clear, though, a narcissist's relationship to sex can also be highly poisonous for anyone in a relationship with them. How that is manifested, depends on the type of narcissist.
One of the authorities on this subject is Sam Vaknin, a self-confessed narcissist who also has a large online presence that he uses to delve deeply into the intricacies of narcissistic behaviour. He has also published a book on the subject, which explores the area in similarly-intricate detail On the subject of sex, he has separated two distinct types of narcissist: the somatic narcissist, and the cerebral narcissist.
"My body is a temple"
The somatic narcissist represents, in some ways, the "classic" perception of what a sex-obsessed narcissist is like: to them, they enjoy the act of sex as a means of conquest and a way to demonstrate their psychological power over the other. The somatic narcissist fits the stereotype of a man (or woman, but most narcissists are men) who goes through members of the opposite sex like chattel, enjoying the act of seduction and "the game" as a way to gain narcissistic supply. Once the victim has been seduced and conquered, the purpose of the "relationship" is over: the somatic narcissist becomes instantly bored, and seeks another victim. This mentality was shown with chilling accuracy by Tom Cruise in the film "Magnolia".
It goes without saying that the somatic narcissist is a misogynist. He uses women as the physical source of narcissistic supply, enjoying the attention he gains from the seduced woman, enjoying the satisfaction of having sexual control over her, acting as "God" in the bedroom. At the same time, however, the sex act itself is meaningless. A sex partner of a somatic narcissist may well enjoy intense and highly-erotic sex, but this will only be part of fulfilling the narcissist's fantasy: the sex partner is a "toy" to be played with, an object to be used. When the narcissist bores of sex with their partner, they will discard them for another at a moment's notice - or simply seek to enhance the "game" through adultery. Both the somatic narcissist and the cerebral narcissist are "auto-erotic" i.e. they gain sexual self-gratification from either themselves or others. The somatic narcissist is the latter, essentially using sex with a partner as a way to masturbate into their partner's body. They do not engage in healthy, intimate sex, but empty, meaningless sex that is simply a method of achieving sexual climax. In this way, the partner of a somatic narcissist is truly "used" as a sex object.
More generally, the somatic narcissist is in love with his own body. For this reason, they will likely be fitness fanatics and gym enthusiasts, following the maxim that "my body is a temple". They are likely to be enthusiasts of cosmetic surgery, wishing to turn themselves into a physical manifestation of their own idealised self. This is all part of the strategy to make themselves (in their eyes) more appealing to the opposite sex, as well as boosting their own self-image. Likewise, a more psychopathic narcissist may also wish to seek to impose their own two-dimensional wishes onto their partner, cajoling them into having cosmetic alterations for the narcissists own pleasure and sense of power. Again, this is another method of gaining narcissistic supply, by treating their partner as an object to be altered and moulded to their desires.
Better than sex
By contrast, the cerebral narcissist sees sex primarily as a chore. While the somatic narcissist is in love with his own body, the cerebral narcissist in love with his mind. For this reason, sexual arousal is mainly an auto-erotic act - the cerebral narcissist usually prefers masturbation to actual sex. The physical act of sex simply doesn't achieve what the cerebral narcissist wants - what he wants is something more than can be found in the act itself. For this reason, cerebral narcissists tend to be porn addicts who look down at actual "sex" as being something beneath them: it may be seen as a "dirty" and base act, while they gain more pleasure from their "superior" minds and exotic sexual fantasies.
That being said, cerebral narcissists are not necessarily celibate. They are as likely to be the kind of person who may well have a long-term partner, and will maintain their physical relationship with them as a method of maintaining their hold on the partner. In this way, fulfilling conjugal duties is the price the cerebral narcissist pays for keeping hold of the narcissistic supply from their partner. At the same time, as the cerebral narcissist is the one who holds the "sex power", he may also enjoy withholding sex rights from their partner as a method of control: if the partner somehow fails to meets his expectations or frustrates him somehow, the cerebral narcissist denies sex to the partner as a form of punishment. Again, this is another way of gaining narcissistic supply, by demonstrating the all-powerful control over the relationship. Like the somatic narcissist, the act of sex itself is meaningless and impersonal: the difference is how these two types of narcissists gain narcissistic supply and sexual satisfaction.
More generally, the cerebral narcissist is likely to be less social and outgoing than the somatic narcissist. As the cerebral narcissist in love with his own mind, it follows that his method of gaining narcissistic supply will also have a more cerebral route: they will seek adulation and attention through their intellectual pursuits, and thus will hold their own intellect to be superior to others' - regardless of the actuality. In this way, while the somatic narcissist will seek to create a fantasy image from his own body, the cerebral narcissist will seek to create a fantasy image from his intellect: imagining himself to be worthy of academic or artistic praise for his work. However, as reality will eventually come to hit him in the face, the result will be a cycle of self-seclusion after meeting some inevitable disappointment. The cerebral narcissist will withdraw into his own self, become more solitary, and convince himself that, like how physical sex is "beneath" him, the outside world is also somehow "unworthy" of his attention. But after some time, like the somatic narcissist. the cerebral narcissist will be in need of another source of narcissistic supply, and thus will re-engage socially.
This is the cycle that the cerebral narcissist goes through; in the same way that the somatic narcissist goes through a cycle of "seduce and destroy", the cerebral narcissist goes through a cycle of engagement and disengagement, acting as leech that seeks the life-blood of attention, attaching to satiate his ego, then disengaging when he prefers the company of his favourite space - his own mind.
As we can see from these two descriptions, both these types of narcissists act as emotional vampires who see society as a vehicle for their own egos. In general, narcissists see their relationships in terms of possession and the need to control. There is also a case to be made for the link between narcissism and other forms of sexuality.
There is also the the social link between narcissism and psychopathy to consider, and what effect this has had on modern society.